1. Field of the Invention
The present invention relates generally to a digital predistortion apparatus and method in a power amplifier. In particular, the present invention relates to a precompensator for linearly amplifying a wideband radio frequency (RF) signal, and an apparatus and method for adaptively controlling the precompensator.
2. Description of the Related Art
In the conventional mobile communication system using RF signals for communication, RF amplifiers are classified into categories including low-power, low-noise receive amplifiers, and high-power transmit amplifiers. In the high-power transmit amplifier, the efficiency of the amplifier, rather than noise, is taken into greater consideration. Accordingly, a high-power amplifier (HPA) typically used in a conventional mobile communication system to achieve high efficiency operates in the vicinity of its nonlinear operating point.
In this case, an output of the amplifier includes an inter-modulation distortion (IMD) component that serves as a spurious signal, not only in its in-band, but also in other frequency bands. In order to delete the spurious component, a feed-forward scheme is typically used. Although the feed-forward scheme can almost completely cancel the spurious component, it has low amplification efficiency and needs to be controlled at an RF stage, thereby increasing the hardware size and system cost.
In the field of mobile communication systems, research into a high-efficiency, low-cost digital predistortion (DPD) scheme is currently being conducted. The digital predistortion scheme calculates an inverse characteristic of nonlinearity of a nonlinear amplifier at a digital stage, and predistorts an input signal using the inverse characteristic, thereby insuring substantial linearity of the output signal of the nonlinear amplifier. The nonlinear characteristics of the nonlinear amplifier can be reclassified into categories including an amplitude modulation-to-amplitude modulation (AM/AM) characteristic, in which the amplitude of an output signal changes depending on the amplitude of an input signal, and an amplitude modulation-to-phase modulation (AM/PM) characteristic, in which the phase of an output signal changes depending on the amplitude of an input signal.
The conventional digital predistorter uses a memory polynomial scheme, simplified from a Volterra scheme, as a linearization scheme for a wideband nonlinear amplifier of a mobile communication system. However, the capability to cancel the nonlinearity of an amplifier is susceptible to a memory order and a polynomial order.
In the memory polynomial-based digital predistortion scheme, the solution can be calculated using an adaptation algorithm. However, as a coefficient order is increased, a convergence speed is decreased, thereby increasing a linearization time of the nonlinear amplifier. Therefore, the convergence speed problem is one of the most important issues that should be resolved in the digital predistortion system.
The conventional Least Mean Square (LMS) algorithm has very low complexity and high stability, but has a very low convergence speed when applied to the DPD scheme. This is because the respective element values of a vector input to the LMS algorithm are dependent on each other, thereby causing a considerable increase in distribution of eigenvalues. As an alternative to the conventional LMS algorithm, there is a method for improving the convergence speed by making the respective element values become independent of each other. However, the alternative method also increases the number of calculations, implementation complexity, and the number of bits, thereby causing an increase in the cost.
Accordingly, a need exists for a system and method for canceling the nonlinearity of an amplifier, and which improves the low-convergence speed problem of the conventional LMS algorithm.